D, shewing the completely closedamnion and the germinal groove.
After the formation of the appendages a fusion takes place between the amnion and serous envelope over a small area close to the head of the embryo.
The whole of the body of the embryo is formed from the ventral plate, and no part from the amnionor serous envelope.
The amnion appears to be usually absorbed before hatching; but in hatching both membranes, if present, are either absorbed, or else ruptured and thrown off.
They appear as a double fold of the blastoderm round the edge of the germinal area, which spreads over the ventral plate, from behind forwards, in a general way in the same manner as the amnion in, for instance, the chick.
The external opening of the invagination of the disc never closes, and after the development of the tube feet begins to widen again, and the amnion to atrophy.
A, a perfectly free passage by which the yolk can pass in between the amnion and serous membrane.
The ventral parts of the amnion and serous membrane have either been ruptured or have disappeared.
These structures are certain embryonic membranes or coverings, which present in their mode of formation and arrangement a startling similarity to the true and falseamnion of the Vertebrata.
In other cases the young develop membranes about themselves--like the amnion of the chick (fig.
It is important to recollect also, as I explained before, that a portion of fluid sometimes exists between the amnion and chorion, which may pass first, and induce the belief that the true waters have escaped, when they have not.
Between these is a gelatinous substance, and within the amnion is a fluid, called the liquor amnii.
Egyptian modes of writing the divine names, say of Amnion Ra or Hathor.
The amnion is nearest the fetus and forms a closed sac around it filled with a fluid known as liquor amnii, in which the fetus floats.
In the uterus the fetus is surrounded by several membranes which are known as the amnion or inner layer, the allantois or central layer, and the chorion or outer layer.
The one layer of the allantois is spread over the outer surface of the amnionand the other over the inner surface of chorion.
The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack.
After this the inner or true amnion becomes severed from the outer or false amnion, though the two sometimes remain connected by a narrow stalk.
The amnion (am) was completely formed, and vitelline vessels had made their appearance.
By the seventh day very obvious movements begin to appear in the amnion itself; slow vermicular contractions creeping rhythmically over it.
The amnion is also large, but the yolk-sack would seem to disappear at an early stage, instead of persisting, as in the Carnivora, till the close of foetal life.
The amnion is a peculiar sack which envelopes and protects the embryo.
The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied.
The inner membrane thus built up forms a completely closed sack round the body of the embryo, and is called the amniotic sack, or amnion proper (fig.
This cavity corresponds with the cavity of the true amnion of higher Vertebrates.
On the fifth day fluid begins to collect in the cavity, and raises the membrane of the amnion to some distance from the embryo.
The true amnion consists of a layer of epiblastic epithelium and generally also of somatic mesoblast, while the false amnion consists, as a rule, of epiblast only; though it is possible that in some cases (the Rabbit?
The edge of this plate coincides with the line of the amniotic fold, and as this fold rises up the two sides of the plate become bent over the embryo and give rise to the inner limb of the amnion or amnion proper.
The formation of the amnion and the allantois and the complete disappearance of the gills are the chief characteristics that distinguish the Amniotes from the lower Vertebrates we have hitherto considered.
The ancestors of these Amniotes develop an amnion and allantois, and gradually assume the mammal, and finally the specifically human, form.
From this the amnion next separates into the envelopes, and the integument upon the embryo.
The foetal water in the amnion is derived from albumen in the ovum; it is here secreted by the oviduct, but in the Mammalia by the internal wall of the uterus, and absorbed by the general envelopes.
In the antagonism of the amnion the integument is evolved, and in its antagonism the branchiæ and lungs.
The amnion is thus the root or primary cyst of the integument.
Mercier has described, and which led him to infer that inflammation of the amnion is the essential cause of the disease.
Cases every now and then occur where a considerable quantity of fluid is found between the chorion and amnion in labour at the full period of pregnancy.
We shall defer the minute description of the amnion and its relations, during the very early periods of utero-gestation, until we describe the embryo.
The amnion contains a fluid known by the name of liquor amnii.
The lobulated appearance is chiefly seen from within, the amnion being raised by a number of irregular convexities.
In five of them "there existed with dropsy of the amnion some malformed or diseased condition of the foetus or its involucra, which rendered it incapable of supporting life subsequent to birth.
We now know from Selenka that the much-discussed ventral pedicle is merely the pedicle of the allantois, combined with the pedicle of the amnion and the rudimentary pedicle of the yelk-sac.
Of the three vesicular appendages of the amniote embryo which we have now described the amnion has no blood-vessels at any moment of its existence.
This envelope takes the name of the germinative membrane, or water-membrane, or amnion (Figure 1.
The yelk-sac is rather smaller, theamnion and chorion larger.
The embryo when about to hatch throws off the egg-shell and amnion in a few seconds.
A remarkable feature in the embryonic development of most Hexapoda is the formation of a protective membrane analogous to the amnion of higher Vertebrates and known by the same term.
Wheeler, however, compares with the "dorsal organ" the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopper Xiphidium.
Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall.
Of the function or origin of the amnion little is known: to state that it is protective affords little explanation.
The embryos of Reptiles, Birds, and Mammals all develop two embryonic or foetal membranes, the amnion and the allantois.
As the allantois could not be absorbed or retracted again into the abdomen, the umbilicus was evolved--that is to say, the scar formed by the union of the folded edge between the body wall and amnion surrounding the stalk of the allantois.
Hence these higher vertebrates (reptiles, birds and mammals) are called Amniota, in contradistinction to fishes and amphibia who have no amnion and are hence known as Anamnia.
These veins originally course in the lateral abdominal wall, close to the fold of the amnion (Fig.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "amnion" in a variety of sentences. We hope that you will now be able to make sentences using this word. Other words: amnion; anatomy; botany; capsule; eardrum; membrane; pellicle; pleura; receptacle; tympanum
